This application claims the priority of German Patent Document DE 101 34 059.1, filed on Jul. 13, 2001, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an oxidation-protected brake disk made of a fiber-reinforced ceramic material, a paint for use on such a brake disk, and a method of producing such a brake disk.
Brake disks made of a fiber-reinforced ceramic material are known from German Patent Documents DE 44 38 455 C1 and DE 198 34 542 A1. German Patent Document DE 44 38 455 C1 shows a method of producing a friction unit, in which a porous carbon body is infiltrated with liquid silicon. In the process, the silicon converts with the carbon to silicon carbide (SiC). The carbon body is preferably constructed as a carbon fiber body. If the friction unit, here, a brake disk, is assembled of two halves, recesses can be made in the carbon bodies before the ceramization, which recesses complement one another upon assembly of the carbon body and form a joint cavity, for example, a cooling duct.
German Patent Document DE 198 43 542 A1 also discloses a method of producing bodies which contain reinforcing fibers. In contrast to German Patent Document DE 44 38 455 C1, fibers, fiber bundles or fiber agglomerates are used and aligned such that the reinforcing fibers in the area of the recesses in the brake disk are oriented approximately parallel to the lateral edges of the recesses. Brake disks made of a fiber-reinforced ceramic material are largely stable with respect to environmental influences. Only a slight wear can be determined during the operation.
One source for various protective paints for carbon materials is Ceramic Solutions, Inc. of Hawthorne, U.S.A. Their primer paint (Designation: CSP-2) is produced on the basis of phosphates and forms an oxidation protection for carbon surfaces. This paint penetrates into the surface porosities and reduces the rate of oxidation. The primer paint is used as a base coat for a cover paint, which is also available from Ceramic Solutions. These cover paints can be used for operating temperatures of up to 927xc2x0 Celsius or 1149xc2x0 Celsius. They have a high stability with respect to water, water vapor and salts of the alkali metals. They are ceramics-based and form a barrier that prevents the oxidation of carbon. They are available in connection with the described primer paint for use on graphite, carbon and carbon/composite materials in order to prevent oxidation.
Ceramic Solutions further offers a protective paint whose operating temperature range is up to 1260xc2x0 C. This paint is extremely stable with respect to water and water vapor and also withstands attacks by salts of the alkali metals. This protective paint is a ceramics-based sealing material that prohibits the oxidation of carbon and other ceramic phases. This protective paint is recommended for use on ceramic composite materials containing carbon or other phases susceptible to active oxidation. This protective paint is recommended as a sealant over a primary protective layer (for example, SiC, Si3N4, and oxides).
It is an object of the invention to reduce or, if possible, to completely avoid damage to the brake disk.
The invention is based on the surprising recognition that, contrary to previous assumptions, brake disks made of fiber-reinforced ceramics, despite the fact that ceramics are very stable per se, require oxidation protection in order to be stable with respect to oxidation under operating conditions, such as those that occur in the case of vehicles.
With respect to the device, it is suggested that the brake disk be provided with a layer of a highly temperature-stable paint which is viscous at the operating temperature of the brake disk. The high temperature stability of the paint is required so that the paint will not be destroyed at temperatures of more than 1,100xc2x0 C. that occur during the braking operation. However, it is an important characteristic of the paint that it becomes viscous in the operating temperature range of the brake disk; that is, that the glass temperature of the paint is below the operating temperature of the brake disk, and the paint is therefore in a viscoplastic to flexible condition until its melting temperature is reached. Advantageously, this behavior has the result that a closed paint film is always formed and the entrance of oxygen to the brake disk is hindered or even prevented. Damage to the paint layer, for example, by the formation of cracks, can also be cured in this manner. It is preferably provided that the paint is temperature-stable to at least 1,200xc2x0 C. and becomes viscous above approximately 400xc2x0 C.
According to a further development of the invention, it may be provided that additional coloring pigments, such as aluminum oxide, be added to the paint. These pigments should have a temperature stability that is comparable to that of the base coat. By using such coloring pigments, it is possible to visually contrast the paint layer from the brake disk and make it visible. For example, brake disks provided with a lacquer coat may thereby be marked in a special manner.
It is particularly advantageous for the coloring pigments to be thermally unstable and to cause a color change when a specific limit temperature is reached. By using such coloring pigments, it is possible to indicate the exceeding of limit temperatures defined by the selection of coloring pigments. Such an indication can be used in order to indicate an overstressing of the brake disk so that, for example, during the next inspection of the vehicle, this overstressing is recognized and an exchange of the brake disks is initiated.
Advantageously, the friction surface of the brake disk as well as the contact surfaces of the brake disks to other components, such as a barrel of the brake disk or fastening elements of the brake disk, are not covered by a paint layer. On the one hand, the frictional characteristics of the brake disk should not be changed; on the other hand, contact surfaces of the brake disk are not freely exposed to oxygen so that the danger of oxygen entry is minimal.
With respect to the use, the object is achieved in that a highly temperature-stable paint, which is viscous at the operating temperature, is applied to a brake disk made of a fiber-reinforced ceramic material.
With respect to the process, it is provided in a supplementary manner that a highly temperature-stable paint, which is viscous at the operating temperature of the brake disk, is applied to a brake disk made of a fiber-reinforced ceramic material. If the friction surface as well as possibly existing contact surfaces are not supposed to be covered by paint, it is also suggested that these surfaces be covered during the application of the paint. According to an alternative embodiment of the invention, it is suggested that these surfaces also be painted and mechanically treated after the painting. This approach has the advantage that additional painting methods can be used, such as dip-coating. This process ensures that paint residues do not reach friction surfaces or contact surfaces, for example, by creeping under a covering. The mechanical stripping of the paint coat may advantageously take place together with a finishing of the friction surfaces or of the contact surfaces.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.